This invention generally relates to a method of preparing epitaxial films for use as infrared detection devices, and more particularly, to lead chalcogenide epitaxial films suitable for photovoltaic applications. Additionally, this invention relates to a simplified method of controlling conductivity type and carrier concentration of the semiconductor material during epitaxial growth.
Thin-films of lead salt alloys have been investigated intensively recently with particular attention to their photovoltaic properties. Special attention has been paid to their possible use as detectors of infrared radiation.
The exploration of vacuum deposition techniques is quite recent and, for the benefit of those who may not be familiar with the pioneering efforts in this art, the following brief bibliography is made of record.
Preparation of Single-Crystal Films Of PbS, by R. B. Schoolar and J. N. Zemel, Journal of Applied Physics, Vol. 35, No. 6, (June, 1964), pp. 1848 to 1851.
Epitaxial Lead-Containing Photoconductive Materials, by R. B. Schoolar, H. R. Riedl, and J. P. Davis, U.S. Pat. No. 3,574,140, (April, 1971).
Method of Preparation of Lead Sulfide PN Junction Diodes, by R. B. Schoolar, U.S. Pat. No. 3,716,424, (February, 1973).
Method Of Varying The Carrier Concentration Of Lead-Tin Sulfide Epitaxial Films, by R. B. Schoolar, U.S. Pat. No. 3,793,070, (February, 1974).
Photoconductive PbSe Epitaxial Films, by R. B. Schoolar and R. J. Lowney, Journal of Vacuum Science Technology, Vol. 8, No. 1, (1971).
More recent efforts although originating from a different direction, include:
Properties Of PbS.sub.1-x Se.sub.x Epilayers Deposited Onto PbS Substrates By Hot-Wall Epitaxy, by K. Duh and H. Preier, Journal of Vacuum Science Technology, pp 1360, (1975).
PbSe Heteropitaxy By The Hot Wall Technique, by K. Duh and H. Preier, Thin Solid Films, Vol. 27, pp 247, (1975).
Double Heterojunction PbS- PbS.sub.1-x Se.sub.x -PbS Laser Diodes With CW Operation Up To 96K, by H. Preier, M. Bleicher, W. Riedel, and H. Maier, Applied Physics Letters, Vol. 28, No. 11, (June, 1976).
PbTe and Pb.sub.0.8 Sn.sub.0.2 Te Epitaxial Films On Cleaved BaF.sub.2 Substrates Prepared By A Modified Hot-Wall Technique, by T. Kasai, D. W. Bassett, and J. Hornung, Journal of Applied Physics, Vol. 47, (July, 1976)
Double-Heterostructure PbS-PbSe-PbS Lasers With CW Operation Up to 120K, by H. Preier, M. Bleicher, W. Riedel, and H. Maier, Journal Of Applied Physics, Vol. 47, (December, 1976).
Growth Of PbTe Films Under Near-Equilibrium, by A. Lopez-Otero, Journal of Applied Physics, Vol. 48, January, 1977).
It is well established that single crystal films of PbS, PbSe, and related compounds, hereinafter referred to as lead salt alloys, can be epitaxially grown on heated alkali halide substrates by vacuum evaporation. It is also known that the conductivity type of these semiconductors in bulk form can be controlled by regulating deviation from stoichiometry. Anion vacancies (lead) make these crystals P-type and cation vacancies make them N-type.
In the past, planar diodes of the lead salts have been produced through the use of various diffusion techniques. The low partial pressures of these materials during sublimation however, means that few collisions occur between their vapor molecules. In the absence of molecular collisions, a film prepared by concurrent sublimation of a set of materials according to the teachings of the present art, with the apparatus disclosed in the Schoolar U.S. Pat. No. 3,716,424 reference for example, would be of non-uniform conductivity, and of graded stoichiometry. An alternative process, exposing a lead compound film to the vapor of a dopant, would only provide a slight change in the conductivity of the film without an appreciable change in its composition. The spectral response of the exposed film would be equal to that of the undoped film.
In an early paper, published in volume 41 of the Journal of Applied Physics, at 3543, (1970), Henry Holloway stated that he used a vacuum deposition technique in which the alkali-halide substrate was maintained at 324.degree. C. Subsequent efforts, (e.g., Properties Of PbS.sub.1-x Se.sub.x Epilayers . . . , by K. Duh and H. Preier, supra) continued to maintain the substrate temperature at or near this value. More recent efforts to improve the electrical properties of epilayer films prepared by vacuum deposition, such as the hot-wall-techniques (HWT), have emphasized purity of materials, cleanliness, and rate of epilayer growth while relying upon unnecessarily complex apparatus without providing device quality films. These efforts have uniformly failed to recognize the desirability of maintaining a near thermodynamic equilibrium at the growth surface of the film, and have therefore allowed the chalcogenide to re-evaporate from the growth surface.